This invention relates to smoke detectors in which a radioactive substance is used in conjunction with two ionisation chambers. Smoke detectors of the kind envisaged include an outer electrode, a collector electrode, and an inner electrode made of or supporting a radioactive substance. The outer electrode and the collector electrode define between them an outer ionisation chamber adapted to allow smoke to enter from the surrounding atmosphere, and the collector electrode and the inner electrode define between them an inner ionisation chamber. The collector electrode has at least one hole capable of passing therethrough radiation emitted by the radioactive substance so as to produce ionisation simultaneously in both ionisation chambers. When a potential difference is maintained across the outer and inner electrodes, the collector electrode assumes an intermediate potential determined by the ratio of ionisation response caused by the radioactive substance in the two chambers. When smoke enters the outer chamber, this ratio, and the potential of the collector electrode, alters and this alteration of potential can be used e.g. to trigger an alarm.
Such detectors are known, and are described for example in British Patent Specification No. 1,280,304 of Hochiki Corporation. FIG. 1 of the accompanying drawings is an axial cross-section through one example of such a detector. An insulating support 10 carries a domed outer electrode 12, an annular collector electrode 14 with an axial hole 16, and a circular inner electrode 18 at the centre of the top face of which is mounted a radioactive substance 20. The outer electrode 12 is maintained at a potential of 9 volts relative to the inner electrode 18 via terminals 22 and 24 attached respectively to the outer and inner electrodes. The radioactive substance 20 emits radiation which causes ionisation of gas in both inner and outer ionisation chambers 26 and 28 respectively. Under the applied electric field, the ions migrate to the electrodes and cause an ion current, typically in the range 10.sup.-10 to 10.sup.-12 Amp, to pass. Under clean air conditions, the collector electrode 14 assumes a potential of, e.g., 5.5 volts. When smoke enters the outer ionisation chamber 28, the smoke particles absorb ions and are too large to migrate rapidly to the electrodes, so that the current is reduced until the potential of the collector has fallen to, e.g., 4.5 volts, the point at which the currents in the outer and inner chambers are again in balance. This fall in potential can be detected via terminal 30 by means of standard electronic circuitry such as a field effect transistor, and used to trigger an alarm.
In the device described by Hochiki Corporation, the only access to the inner ionisation chamber 26 is via the hole 16, which is made so small as substantially to prevent the ingress of smoke particles. This feature gives rise to a number of difficulties and disadvantages in design. The feature is also quite unnecessary, since the inner ionisation chamber can readily be designed so that the ion current is substantially unaffected by the ingress of smoke particles.
The detector is designed such that ions in the inner ionisation chamber are collected at the electrodes after only a short passage. Moreover the ions are collected rapidly because the electric field in the inner ionisation chamber is high, and the chamber operates under essentially saturated ion current conditions, that is to say, such that most of the ions produced by the ionising radiation in the chamber are collected at the electrodes; whereas the outer ionisation chamber 28 operates under unsaturated conditions.